LDAP is the Lightweight Directory Access Protocol, and is a protocol
used to access "Directory Servers". The Directory is a special kind
of database that holds information in a tree structure.

The concept is similar to your hard disk directory structure, except
that in this context, the root directory is "The world" and the
first level subdirectories are "countries". Lower levels of the
directory structure contain entries for companies, organisations
or places, while yet lower still we find directory entries for
people, and perhaps equipment or documents.

The PHP LDAP library allows you to query and set values in remote
and local LDAP databases.
